Mr Gao, 50, had been imprisoned on and off since 2006, including some periods in extrajudicial facilities without charge. He has said in interviews that he was at times tortured.

He attained international publicity for his campaigning for religious freedom, particularly for members of the banned religious group Falun Gong.

Mr Gao's lawyer, Jared Genser, told Asia Pacific that his client became a target of the authorities when he began defending the rights of persecuted and vulnerable groups in China.

"Interestingly, he actually was ranked by the Chinese Ministry of Justice as one of the top ten corporate lawyers in China, this was in the early 2000s," Mr Genser said.

"He saw the injustice of the judicial system where the best legal arguments didn't prevail but that people bought their own justice.

"He started to have a parade of people come to him asking for help, people whose land was taken without compensation, people who were victims of environmental disasters in their own backyards, later on religious Christians who were being persecuted without justification.

"Ultimately he crossed the red line for China when he started to represent Falung Gong practitioners who had been tortured horrifically by the Chinese Government."

China has not commented on the specifics of Mr Gao's case saying it was a domestic matter, but authorities have said that torture to extract confessions is illegal.

Mr Gao was cut off from his family during his time in prison, said Maya Wang of Human Rights Watch.

"His family only saw him twice in the last three years," she said.

He saw the injustice of the judicial system where the best legal arguments didn't prevail but that people bought their own justice.

Jared Genser, managing director of Perseus Strategies

"Every time, they were only allowed to talk about conditions at home, and not allowed to ask about his health."

Ms Wang said there was concern that like other activists, he could find himself released from prison only to be put under house arrest.

Chinese President Xi Jinping's administration has cracked down on dissent, detaining and jailing activists, muzzling internet critics and strengthening restrictions on journalists in what some rights groups call the worst suppression of free expression in recent years.

China arrested prominent lawyer Pu Zhiqiang in June in a case that sparked outcry among rights groups in both China and the west.

Many lawyers and journalists were detained in the run-up to the anniversary of the bloody suppression of pro-democracy protests at Tiananmen Square in 1989.